Process for the preparation of lithium hexafluoroarsenate

ABSTRACT

The present invention provides a novel thermal solid state method for the synthesis of lithium hexfluoroarsenate either in a two step or a single step method, preferably at last substantially in the solid state.

FIELD OF THE INVENTION

The present invention relates to a novel process for the preparation oflithium hexafluoro arsenate (LiAsF₆) useful as an electrolyte innonaqueous solid state and polymer electrolyte lithium based primary andsecondary batteries.

BACKGROUND OF THE INVENTION

Lithium hexafluoro arsenate (LiAsF₆) is useful as an electrochemicalenergy producing electrolyte cell with small cation and large anionwhich never helps good conductivity in non conventional media likenon-aqueous solvents, solid state ionic materials and polymer basedelectrolyte medium. The general method, available in literature dealswith AsF₅, F₂ etc., which are difficult to handle. A few chemicals ofthe above type need sophisticated equipment to handle, operate andinstallation needs heavy investment. Apart from that environmentallysuch unfriendly chemicals need protection.

A literature survey revealed the following references:

1. Fluorine Chemistry Vol II J. H. Simons Ed. Academy Press New York,pg. 38, 1955.

2. Fluorine Chemistry Vol H. J. Emedius Ed. Academic Press New York page76 (1955).

3. Fluorine Chemistry Vol II W. Large Ed. Academic Press New York page12(1955).

4. P. L. Soni. Textbook of Inorganic Chemistry S. Chand & Co. Delhi page425 (1985).

Lithium hexafluro arsenate (LiAsF₆) is prepared by all solid statethermal or partial solid state reactions where arsenous trioxide (As₂O₃)is allowed to react with lithium hydroxide or lithium oxide or lithiumcarbonate or lithium nitrate in equimolar proportions under controlledthermal conditions so as to get lithium arsenite (LiAsO₂) which wasallowed to react with HF or NH₄F so as to obtain LiAsF₆. Alternatively,arsenous trioxide (As₂O₃) is mixed with LiOH or Li₂O or Li₂CO₃ and NH₄Fis required proportions and the mix is heated in an electric furnace inbetween temperatures 150-300° C. for 4 hours continuously. The resultantproduct was examined by X-ray for purity and identity.

Lithium hexafluoro arsenate (LiAsF₆) prepared by other methods sufferfrom the following disadvantages.

1. Toxic chemicals are required for preparation.

2. Difficult to handle toxic chemicals.

3. The product contains unreacted reactants as impurity

4. Partial reactions occur.

5. Needs several equipment to avoid pollution problems.

6. It is not based as solid state reactions.

OBJECTS OF THE INVENTION

The main object of this present invention is to provide a novel methodfor the preparation of lithium hexafluoro arsenate (LiAsF₆) whichobviates the draw backs mentioned above.

Another object of this present invention is to provide a complete solidstate single/double stage preparation or partial solid state method ofpreparation.

Still yet another object of this invention is to avoid costly equipmentsto handle obnoxious gases used for the preparation of lithium hexafluoroarsenate (LiAsF₆).

Yet still another object of this invention is to develop a singlethermal procedure for the preparation of lithium hexafluoro arsenate(LiAsF₆) or an intermediate lithium hypoarsenite (LiAsO₂).

Another object of this present invention is to get the product withoutany side reaction or partial reaction.

SUMMARY OF THE INVENTION

Accordingly the present invention provides a process for the preparationof lithium hexafluoro arsenate comprising contacting arsenous trioxidewith a lithium source and a fluorine source, all being in solid stateand heating to obtain lithium hexafluoro arsenate.

In one embodiment of the invention, the lithium source is selected fromthe group consisting of LiOH, Li₂O, LiNO₃ and Li₂CO₃.

In another embodiment of the invention, the fluorine source is selectedfrom HF and NH₄F.

In one embodiment of the invention, the process comprises two stagescomprising mixing the lithium source with arsenous trioxide in equimolarproportions and heating the slowly in an electric furnace initially to100° C. and then continuously for 4 hours at a temperature in the rangeof 200-300° C. to obtain lithium meta arsenate (LiAsO₂), reacting thelithium metaarsenite product with a fluorine source for about 4 hours toobtain lithium hexafluoro arsenate.

In a further embodiment of the invention, the fluorine source isselected from 10-30% HF and 6-9 times the molecular weight of NH₄F.

In another embodiment of the invention, lithium meta arsenate (LiAsO₂)is mixed with ammonium fluoride and heated in an electric furnace inbetween temperatures of 150-300° C. continuously for 4 hours to obtainLiAsF₃ confirmed by X-ray analysis.

In another embodiment of the invention, the reaction is carried out in asingle step process comprising mixing a lithium source selected from thegroup consisting of LiOH, Li₂O, Li₂CO₃ and LiNO₃ with equimolar quantityof arsenous trioxide (As₂O₃) and seven times molecular weight ammoniumfluoride, heating the solid mixture in an electric furnace at atemperature in the range of 150-300° C. for 4 hours to obtain thedesired product.

In one embodiment of the invention, the method comprises a full solidstate thermal reaction of lithium salt with arsenous trioxide (As₂O₃)and ammonium fluoride mixture in the ratio 1:1:6-9 heated in an electricfurnace continuously for 4 hours at a temperature in the range of150-300° C. to obtain lithium tetrafluoro arsenate confirmed by X-rayanalysis.

In an embodiment of the invention LiOH or Li₂O or Li₂CO₃ or LiNO₃ ismixed with arsenous trioxide in the ratio 1:1 or get LiAsO₂.

In another embodiment of the invention LiAsO₂ is mixed with NH₄F in aratio 1:6-9.

In yet another embodiment of this invention LiAsO₂ is mixed with 10-30%HF.

In yet another embodiment of this invention lithium salt, arseneoustrioxide and NH₄F are mixed in the ratio 1:1:6-9.

In still yet another embodiment of this invention an electric furnace isused to prepare LiAsO₂ intermediate or the final product LiAsF₆.

In another embodiment of this reaction all solid state or partial solidsate reaction is possible depending on the reactions envisaged forgetting the end product LiASF₂.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

FIG. 1 shows the X-ray analysis of the product lithium hexafluoroarsenate obtained by the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a novel thermal solid state method forthe synthesis of lithium hexafluoroarsenate either in a two step or asingle step method. A lithium salt such as pure dry and AR LiOH or Li₂Oor Li₂CO₃ or LiNO₃ is mixed with equimolar proportion of arseneoustrioxide and heated in an electric furnace carefully at temperaturesbetween 250-400° C. for 4 hours continuously. The resultant intermediateproduct was mixed with six to nine times its molecular weight ofammonium fluoride (AR) and carefully heated in a muffle furnacecontinuously for 4 hours at a temperature in between 150-300° C. toobtain LiAsF₆. The following reactions occur when As₂O₃ reacts with Li₂Oor Li₂CO₃ or LiOH or LiNO₃.

Solid State Reaction

1. 2LiOH+As₂O₃→2LiAsO₂+H₂O

2. 2LiNO₃+As₂O₃→2LiAsO₂+2NO₂+O

3. Li₂CO₃+As₂O₃→2LiAsO₂+CO₂

4. Li₂O+As₂O₃→LiAsO₂

LiAsF₆ can be obtained by reacting the intermediate product with 10times its molecular weight of NH₄F. In another alternative route, theintermediate product lithium hypo arsenite reacts with either 10-30% HFor 7 to 9 times the quantity of N H₄F to get LiAsF₆ product.

LiAsO₂+6HF→LiAsF₆=2H₂O+H₂

A mixture of LiAsO₂ and NH₄F of equimolar proportion 1:(6-9) reactthermally to form LiAsF₆.

LiAsO₂+6NH₄F→LiAsF₆+2NH₃+H₂O+H₂

Also a mixture of As₂O₃ and lithium salt and ammonium fluoride reacts asfollows

Li₂O+As₂O₃+12NH₄F→2LiAsF₆+12NH₃+4H₂O+2H₂

The resultant product lithium tetrafluoro arsenate was confirmed byX-ray analysis. The reactions are preferably carried out in an electricfurnace. The reaction can be either one step or two step depending onthe reactions envisaged for getting the final product LiAsF₆. Pure dryAR LiOH or Li₂O or Li₂CO₃ or LiNO₃ is mixed with equimolar proportionsof arseneous trioxide and heated the mixture in a electric furnacecarefully in between the temperatures 250-400° C. for 4 hourscontinuously. The resultant intermediate product was mixed with six tonine times the molecular of ammonium fluoride (AR) and the mix wascarefully heated in a muffle furnace continuously for 4 hours in betweentemperatures 150-300° C. and the resultant product LiAsF₆.

The following examples are given by way of illustration and thereforeshould not be construed to limit the scope of the present invention.

EXAMPLE 1

An equimolar mixture of Li₂O (AR) and As₂O₃ (AR) are ground well andthen heated to 200° C. in an electric furnace for 4 hours continuously.The product was ground well and was mixed with 7 times the weight ofNH₄F (AR). Then this mix was again heated in an electric furnace for 4hours continuously at a temperature of 200° C. to get LiAsF₆.

Components Composition Li₂O 0.30 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4 hoursTemperature 200° C. Nature of Product Transparent Efficiency of theprocess >90%

EXAMPLE 2

An equimolar mixture of LiOH (AR) and As₂O₃ (AR) are ground well and themix heated to 200° C. in an electric furnace for 4 hours continuously.The product was ground well and was mixed with 7 times the weight ofNH₄F (AR). The mix was then again heated in an electric furnace for 4hours continuously at a temperature of 200° C. to get LiAsF₆.

Components Composition LiOH 0.24 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4 hoursTemperature 200° C. Nature of the product Transparent Efficiency of theprocess >91%

EXAMPLE 3

An equimolar mixture of Li₂CO₃ (AR) and As₂O₃ (AR) are ground well andthe mix heated to 200° C. in an electric furnace for 4 hourscontinuously. The product was ground well and was mixed with 7 times theweight NH₄F (AR). The mix was then again heated in an electric furnacefor 4 hours continuously at a temperature of 200° C. to get LiAsF₆.

Components Composition Li₂CO₃ 0.74 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4Hours Temperature 200° C. Nature of the product Transparent Efficiencyof the process >90%

EXAMPLE 4

An equimolar mixture of LiNO₃ (AR) and As₂O₃ (AR) are ground well andthe mix heated to 200° C. in an electric furnace for 4 hourscontinuously. The product was ground well and was mixed with 7 times theweight of NH₄F (AR) and then this mix was again heated in an electricfurnace for 4 hours continuously at a temperature of 200° C. to getLiAsF₆.

Components Composition LiNO₃ 0.69 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4hours Temperature 200° C. Nature of product Transparent Efficiency ofthe process >91%

EXAMPLE 5

AR quality of Li₂O with AR arseneous trioxide and AR NH₄F of ratios1:1:(6-9) are ground well to get uniform mix which was heated to 200° C.to get the product LiAsF₆.

Components Composition Li₂O 0.30 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4 hoursTemperature 200° C. Nature of the product Transparent Efficiency of theprocess >90%

EXAMPLE 6

AR quality of Li₂CO₃ with AR arseneous trioxide and AR NH₄F of ratios1:1:(6-9) are ground well to get uniform mix which was heated to 200° C.to get the product LiAsF₆.

Components Composition Li₂CO₃ 0.74 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4hours Temperature 200° C. Nature of the product Transparent Efficiencyof the process >90%

EXAMPLE 7

AR quality of LiOH with AR arseneous trioxide and AR NH₄F of ratios1:1:(6-9) are ground well to get uniform mix which was heated to 200° C.to get the product LiAsF₆.

Components Composition LiOH 0.24 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4 hoursTemperature 200° C. Nature of the product Transparent Efficiency of theprocess >90%

EXAMPLE 8

AR quality of LiNO₃ with AR arseneous trioxide and AR NH₄F of ratios1:1:(6-9) are ground well to get uniform mix which was heated to 200° C.to get the product LiAsF₆.

Components Composition LiNO₃ 0.69 g As₂O₃ 1.98 g NH₄F 2.59 g Time 4hours Temperature 200° C. Nature of the product Transparent Efficiencyof the process >90%

CONCLUSION

1. Lithium salts like Li₂O, Li₂CO₃, LiOH, LiNO₃ or the mixtures reactwith arseneous trioxide in the ratio 1:1 to get the intermediatecompound LiAsO₂.

2. LiAsO₂ reacts with AR NH₄F to get LiAsF₆.

3. LiAsO₂ reacts with AR HF (10-30%) to get LiAsF₆.

4. AR lithium salts, As₂O₃ and NH₄F (AR) in the ratio 1:1:(6-9) react togive LiAsF₆.

5. The temperature range is between 10° C. to 600° C. depending on thereaction processes.

6. The time of heating is in the range of 2-4 hours.

The main advantages of the present invention are:

1. A single step solid thermal method or a two step semi solid statereaction are possible depending on conditions of reactions.

2. Obnoxious gases and liquids like AsF₃, AsF₅, F₂ are avoided, therebyavoiding pollution problems.

3. Costly equipment to handle these gaseous materials are avoided.

4. A simple solid state/semi solid state thermal process is developed toprepare LiAsF₆.

We claim:
 1. A process for the preparation of lithium hexafluoroarsenate comprising contacting arsenous trioxide with a lithium sourceand ammonium fluoride, all being in solid state and heating to obtainlithium hexafluoro arsenate.
 2. A process as claimed in claim 1 whereinthe lithium source is selected from the group consisting of LiOH, Li₂O,LiNO₃ and Li₂CO₃.
 3. A process as claimed in claim 1 wherein the lithiumsource, the arsenous trioxide and the ammonium fluoride are used in aratio of 1:1:6-9.
 4. A process as claimed in claim 1 wherein the processis carried out in a single thermal step comprising mixing the lithiumsource selected from the group consisting of LiOH, Li₂O, Li₂CO₃ andLiNO₃ with arsenous trioxide (As₂O₃) and ammonium fluoride in a ratio of1:1:6-9, and heating the resulting solid mixture in an electric furnaceat a temperature in the range of 150-300° C. for 4 hours to obtainlithium hexafluoroarsenate.
 5. A process as claimed in claim 1 whereinthe lithium source and arsenous trioxide (As₂O₃) and ammonium fluorideare mixed in a ratio 1:1:6-9, heated in an electric furnace continuouslyfor 4 hours at a temperature in the range of 150-300° C. to obtainlithium hexafluoro arsenate confirmed by X-ray analysis.
 6. A process asclaimed in claim 1 wherein the process comprises stages comprising firstmixing the lithium source with arsenous trioxide in equimolarproportions and heating slowly in an electric furnace initially to 100°C. and then continuously for 4 hours at a temperature in the range of200-300° C. to obtain lithium metaarsenate (LiAsO₂), and secondlyreacting the lithium metaarsenate product with ammonium fluoride forabout 4 hours to obtain lithium hexafluoro arsenate.
 7. A process asclaimed in claim 6 wherein the ratio of the lithium metaarsenate to theammonium fluoride is 1:6-9.
 8. A process as claimed in claim 6 whereinthe lithium source is selected from the group consisting of LiOH or Li₂Oor Li₂CO₃ or LiNO₃ and is mixed with arsenous trioxide in a molar ratioof 1:1 to obtain an intermediate product LiAsO₂.
 9. A process as claimedin claim 8 wherein the ratio of the intermediate LiAsO₂ obtained to theammonium fluoride is 1:6-9.
 10. A process as claimed in claim 8 whereinan electric furnace is used to prepare the LiAsO₂ intermediate.
 11. Aprocess as claimed in claim 1 wherein the temperature is from 150° to300° C.
 12. A process as claimed in claim 8 wherein the intermediateLiAsO₂ is prepared at a temperature in the range of 200-600° C.
 13. Aprocess for the preparation of lithium hexafluoroarsenate comprisingmixing a lithium source with arsenous trioxide (As₂O₃) and ammoniumfluoride in a ratio of 1:1:6-9, heating the resulting solid mixture inan electric furnace at a temperature in the range of 150-300° C. for 4hours to obtain the lithium hexafluoroarsenate product.
 14. A process asclaimed in claim 13 wherein the lithium source is selected from thegroup consisting of LiOH, Li₂O, LiNO₃ and Li₂CO₃.
 15. A process for thepreparation of lithium hexafluoroarsenate comprising first mixing alithium source with arsenous trioxide in equimolar proportions andheating slowly in an electric furnace initially to 100° C. and thencontinuously for 4 hours at a temperature in the range of 200-300° C. toobtain lithium meta arsenate (LiAsO₂), and secondly reacting the lithiummetaarsenate product with ammonium fluoride for about 4 hours to obtainlithium hexafluoro arsenate.
 16. A process as claimed in claim 15wherein the ratio of the lithium metaarsenate to the ammonium fluorideis 1:6-9.
 17. A process as claimed in claim 15 wherein the lithiumsource is selected from the group consisting of LiOH, Li₂O, LiNO₃ andLi₂CO₃.